3 Answers
3

If fact, there is no concrete-and-simple rule that would say, whether -а/-я or -у/-ю should be used for genitive singular of a specific second-declension noun — people generally just intuitively remember endings.

The very imprecise rule is that nouns for more concrete things are often -а/-я, while more abstract things are often -у/-ю — but that's very imprecise. Or, sometimes people say something like: "If I can see/imagine it as a whole, it probably has -а/-я ending, otherwise — -у/-ю." Also very imprecise.

But, like in any language, almost every item of this "rule" has own set of exceptions. You can see more about this in that article. Therefore genitive endings are usually just remembered on a per-word basis.

When use -у, -ю contains weird phrases (not fixed after Google Translate). E.g.: with the value space, space, etc., oznachayuschie, difficult. You can try combining you answer with parts of my translation (it's not ideal either, but in some points it may be better — in some not).
– Sasha♦Mar 30 '17 at 22:06

1

Thanks, something fixed. I took some last part of text from you.
– stegetsjMar 30 '17 at 22:21

Your reference uses animate and inanimate denominations for nouns, but the classic Ukrainian grammar does not qualify nouns for this. We use grammatical gender instead (Masculine, Feminine, and Neuter).